The present invention pertains to the field of cryogenic air separation, and in particular to a process for the production and delivery of intermediate pressure oxygen from a cryogenic air separation plant.
There are typically two ways of delivering the oxygen produced from a cryogenic air separation plant. Historically, oxygen product was withdrawn as a vapor from the bottom of the lower pressure column of a double-column distillation system, warmed to ambient temperature, and either delivered to the user at very low pressure or compressed. This type of process is commonly referred to as a GOX-plant
The maximum oxygen pressure that can be realized when oxygen is withdrawn as a vapor from the lower pressure column is severely limited. This is due to the desire to operate the lower pressure column at a pressure as close to atmospheric pressure as possible to maintain efficient operation. The maximum oxygen delivery pressure also has been reduced further by the recent use of low pressure drop structured packing. In practice, the maximum, efficient, oxygen delivery pressure is only around 17 psia when oxygen is withdrawn as a vapor from a lower pressure column near atmospheric pressure. A supplemental product compressor may be justified for oxygen pressures greater than about 17 psia.
Many disclosures in the literature are directed at improving the efficiency of oxygen producing plants that produce oxygen as a vapor from a lower pressure column. U.S. Pat. No. 5,669,237 (Voit) is one example which is applicable to the production of low purity oxygen. A notable feature of this patent is the use of a portion of feed air to provide boilup to the bottom of the lower pressure column.
More recently, it has become commonplace to withdraw liquid oxygen from the lower pressure column, raise the pressure of the oxygen by using either static head or a pump, and vaporize the oxygen by condensing some suitably pressurized stream. This method of oxygen delivery is referred to as LOX-Boil or pumped-LOX. An example of LOX-Boil is taught in U.S. Pat. No. 4,560,398 (Beddome, et al.); an example of pumped-LOX is taught in U.S. Pat. No. 5,355,682 (Agrawal, et al.).
Oxygen delivery using LOX-Boil or pumped-LOX is commonly accomplished by condensing a portion of the incoming pressurized air. The source for the pressurized air is the discharge of a main air compressor. Since the discharge pressure of the main air compressor is set by the operating pressure of the higher pressure column, a lower bound on the condensing air pressure is established. As a result, the lowest pressure at which oxygen may be produced efficiently is approximately 23 psia. Of course, oxygen may be produced efficiently at pressures greater than 23 psia by using a booster compressor to raise the pressure of the condensing air stream. The absolute lowest efficient pressure may vary somewhat from 23 psia, depending on many factors such as: pressure of the lower pressure column, pressure drop in the distillation columns, heat exchanger temperature approaches, feed and product pressure drops, etc.
Many disclosures in the literature are directed at improving the efficiency of LOX-Boil and pumped-LOX plants. One example is U.S. Pat. No. 5,355,681 (Xu), which is applicable to the coproduction of liquid products. A key feature of one of the embodiments (as illustrated in FIG. 1 of U.S. Pat. No. 5,355,681) is the use of a portion of feed air to provide boilup to the bottom of the lower pressure column.
It is desired to provide an efficient process for producing oxygen from a cryogenic air separation plant at a pressure intermediate that which is achievable by either withdrawing vapor from the lower pressure column or by vaporizing liquid oxygen against a stream of air which is nominally at the pressure of the higher pressure column.